Passive intermodulation (PIM), commonly referred to as “rusty bolt effect,” is the generation of unwanted signals by the non-linear mixing of two or more frequencies in a passive device, such as a connector or cable, that normally behaves linearly but when subject to high radio frequency (RF) power tones behaves nonlinearly. Non-linear behavior affecting a system can result, for example, from a loose or corroded connector or from the close proximity of the system to oxidized metal. Non-linear behavior affecting a system emitting RF signals can also result from the re-radiation of emitted signals from sources outside of the system, such as unrelated antennas, lighting fixtures, HVAC ducts, exposed metal such as roof flashing, unexposed metal and fasteners hidden beneath roofing tar, etc.
PIM has surfaced as a particular problem for cellular systems. If the generated PIM harmonic frequency components fall within the receive band of a base station of a cellular site, they can effectively block a channel and make the base station receiver think that a carrier signal is present when one is not. Generally the harmonics of concern are third, fifth, and seventh order, where the third order is of greatest signal strength, and therefore, of primary concern.
Test systems are available to measure PIM by creating signals at different frequencies, amplifying and combining them, and providing them to components of a cellular system and measuring a reflected signal generated by a PIM source within the components. Once a PIM source is identified, PIM can be reduced or eliminated by replacing or removing the PIM source. For example, PIM is commonly resolved by replacing faulty cables or connectors, or by removing re-radiating sources.
While such test systems may be effective for identifying PIM sources within a cellular system, other sources of PIM existing outside of the cellular system such as the environment surrounding the cellular system are not identifiable by measuring the response of the components internal to the cellular system. Test systems for measuring PIM can be connected via a test port to a transmit/receive antenna to detect PIM beyond the antenna and to approximate a distance to the source of the PIM. However, while the distance can be approximated, the direction may not be easily determined with a typical antenna usable for PIM detection. Further, such test systems can be bulky and high powered, making them impractical for mobile use in tracking down PIM sources in the field, external to the affected system.